Method for manufacturing piezoelectric device with a composite piezoelectric substrate

ABSTRACT

A piezoelectric device is manufactured in which the material of a supporting substrate can be selected from various alternative materials. Ions are implanted into a piezoelectric substrate to form an ion-implanted portion. A temporary supporting substrate is formed on the ion-implanted surface of the piezoelectric substrate. The temporary supporting substrate includes a layer to be etched and a temporary substrate. The piezoelectric substrate is then heated to be divided at the ion-implanted portion to form a piezoelectric thin film. A supporting substrate is then formed on the piezoelectric thin film. The supporting substrate includes a dielectric film and a base substrate. The temporary supporting substrate is made of a material that produces a thermal stress at the interface between the temporary supporting substrate and the piezoelectric thin film less than the thermal stress at the interface between the supporting substrate and the piezoelectric thin film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a compositepiezoelectric substrate including a piezoelectric thin film and apiezoelectric device including the composite piezoelectric substrate.More particularly, the present invention relates to a method formanufacturing a composite piezoelectric substrate in which apiezoelectric thin film is subjected to heat treatment and apiezoelectric device including the composite piezoelectric substrate.

2. Description of the Related Art

Various piezoelectric devices that utilize piezoelectric thin films arebeing developed.

Piezoelectric devices include a composite piezoelectric substrateincluding a piezoelectric thin film and a supporting substrate (see, forexample, Japanese Unexamined Patent Application Publication No.2002-534886).

In accordance with Japanese Unexamined Patent Application PublicationNo. 2002-534886, the supporting substrate may be made of sapphire,silicon, or gallium arsenide, and the piezoelectric thin film may bemade of a piezoelectric substance, such as quartz, lithium tantalate(LT), or lithium niobate (LN). Japanese Unexamined Patent ApplicationPublication No. 2002-534886 also discloses a method for manufacturing acomposite piezoelectric substrate in which a piezoelectric thin film isformed by dividing a piezoelectric substrate after ion implantation.

In this method, the piezoelectric substrate has a thickness sufficientfor bonding, and ions, such as hydrogen ions, are implanted into onemain surface of the piezoelectric substrate to form an ion-implantedlayer. This main surface is then bonded to a supporting substrate byactivated bonding or affinity bonding. The piezoelectric substrate isthen heated to be divided at the ion-implanted layer to form thepiezoelectric thin film.

Such a method can produce a composite piezoelectric substrate includinga very thin piezoelectric film supported by a supporting substrate.However, implanted ions remaining in the piezoelectric thin filmadversely affect the piezoelectricity of the composite piezoelectricsubstrate. Thus, in order to recover the piezoelectricity of thecomposite piezoelectric substrate, the piezoelectric thin film issometimes heated at a temperature greater than the dividing temperaturefor a long period of time to remove the residual ions from thepiezoelectric thin film.

Thus, in the method for manufacturing a composite piezoelectricsubstrate in which a piezoelectric thin film is formed by dividing apiezoelectric substrate at an ion-implanted layer, the piezoelectricthin film bonded to a supporting substrate is heated to a dividingtemperature and an annealing temperature as described above. Duringthese heating processes, a high thermal stress at the interface betweenthe supporting substrate and the piezoelectric thin film can causeproblems, such as the detachment or cracking of the piezoelectric thinfilm. Such defects in the piezoelectric thin film are particularlynoticeable during the manufacture of large composite piezoelectricsubstrates, making it difficult to commercially manufacture largecomposite piezoelectric substrates. To avoid this, a supportingsubstrate must be made of a material that produces a low thermal stressduring heat treatment. This imposes a significant constraint on thecoefficient of linear expansion of the material.

Since the functions of piezoelectric devices depend on thecharacteristics of a supporting substrate, it is desirable to havevarious alternative suitable materials for the supporting substrate. Inthe case of devices for filter applications, a reduction in thecoefficient of linear expansion of a supporting substrate can improvethe temperature-frequency characteristics of a filter. However, due tothe constraint on the coefficient of linear expansion, the material of asupporting substrate cannot have a coefficient of linear expansion muchless than the coefficient of linear expansion of a piezoelectric thinfilm. It is desirable that the material of a supporting substrate havehigh thermal conductivity to improve the heat radiation characteristicsand the electric power resistance of the supporting substrate. It isalso desirable that the material of a supporting substrate beinexpensive in order to reduce the manufacturing costs of devices.However, such a material does not always satisfy the constraint on thecoefficient of linear expansion. Furthermore, use of a material havinghigh processibility, such as silicon, for a supporting substrate enablesthe supporting substrate to have a complicated structure. This permits amethod for forming a piezoelectric thin film by dividing a piezoelectricsubstrate at an ion-implanted layer to be applied to various devices,such as micro-electro-mechanical systems (MEMS) and gyros. However, sucha high-processibility material rarely satisfies the constraint on thecoefficient of linear expansion. Thus, the material of a supportingsubstrate is significantly limited.

In accordance with Japanese Unexamined Patent Application PublicationNo. 2002-534886, electrodes are formed on the divided surface of theion-implanted layer to produce a surface acoustic wave device. In thiscase, even after the piezoelectricity recovery treatment, ions remainingin the vicinity of the divided surface can cause significantpiezoelectric degradation.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a method for manufacturing a piezoelectricdevice in which various alternative materials can be used for asupporting substrate, and a piezoelectric device having improvedcharacteristics.

In accordance with a preferred embodiment of the present invention, amethod for manufacturing a composite piezoelectric substrate thatincludes a piezoelectric thin film supported by a supporting substratepreferably includes an ion implantation step of implanting an ionizedelement into a piezoelectric substrate to form a portion having a peakconcentration of the ionized element, a temporary supporting step offorming a temporary supporting substrate on a side of the ion-implantedsurface of the piezoelectric substrate, the temporary supportingsubstrate being made of the same material as the piezoelectric substrateor producing a thermal stress at the interface between the temporarysupporting substrate and the piezoelectric substrate that is less thanthe thermal stress at the interface between the supporting substrate andthe piezoelectric substrate, a heating step of heating the piezoelectricsubstrate to divide the piezoelectric thin film from the piezoelectricsubstrate at the portion having a peak concentration of the ionizedelement to form the piezoelectric thin film, and a supporting step offorming the supporting substrate on the piezoelectric thin film.

In accordance with this manufacturing method, the compositepiezoelectric substrate including the piezoelectric thin film supportedby the supporting substrate can be manufactured such that thepiezoelectric thin film is formed by dividing the piezoelectricsubstrate at the portion having a peak concentration of the implantedionized element. The temporary supporting substrate preferably producesa negligible thermal stress at the interface between the temporarysupporting substrate and the piezoelectric thin film. Alternatively, thetemporary supporting substrate may preferably produce a thermal stressat the interface between the temporary supporting substrate and thepiezoelectric thin film that is less than the thermal stress at theinterface between the supporting substrate and the piezoelectric thinfilm. Since the heating step is performed while the temporary supportingsubstrate is disposed on a side of the ion-implanted surface of thepiezoelectric substrate, the heating step produces a reduced thermalstress at the interface between the piezoelectric thin film and thetemporary supporting substrate. The thermal stress at an interfacedepends on the difference in the linear expansions on both sides of theinterface on the assumption that no restriction exists on the interface.Since the supporting substrate is formed on the piezoelectric thin filmafter the division at the portion having a peak concentration of theionized element implanted in the piezoelectric substrate and after aheating process for recovering piezoelectricity, the supportingsubstrate may be made of a material having any coefficient of linearexpansion.

In the supporting step, the supporting substrate is preferably formed ona side of the divided surface of the piezoelectric thin film. Inaccordance with a preferred embodiment of the present invention, thesupporting step is preferably followed by a temporary supportingsubstrate removing step of removing the temporary supporting substratedisposed on the side of the ion-implanted surface of the piezoelectricthin film and an electrode forming step of forming a functionalelectrode of a piezoelectric device on the side of the ion-implantedsurface.

In this manufacturing method, the front and back sides of thepiezoelectric thin film are on opposite sides to the front and backsides of piezoelectric thin films manufactured by conventional methods.More specifically, preferably, the divided surface of the piezoelectricthin film faces the supporting substrate, and the ion-implanted surfaceof the piezoelectric thin film is spaced away from the supportingsubstrate. In the piezoelectric thin film formed by dividing thepiezoelectric substrate at the portion having a peak concentration ofthe implanted ionized element, local piezoelectric degradation tends todecrease with decreasing the density of residual implanted ions. Thus,piezoelectric degradation is less in the vicinity of the ion-implantedsurface than in the vicinity of the divided surface of the portionhaving a peak concentration of the implanted ionized element. Thus, apiezoelectric device having improved characteristics can be manufacturedby forming a functional electrode on the side of the ion-implantedsurface that suffers less piezoelectric degradation.

In accordance with a preferred embodiment of the present invention, thetemporary supporting substrate preferably includes a layer to be etched,and the temporary supporting substrate removing step preferably includesremoving the temporary supporting substrate by etching the layer to beetched.

Thus, the temporary supporting substrate can be removed by etchingwithout unnecessary stress or impact on the piezoelectric thin film.This can further reduce defects in the piezoelectric thin film. Thus, apiezoelectric device having consistent characteristics can bemanufactured. Furthermore, this permits the reuse of a temporarysupporting substrate main body after the layer to be etched has beenremoved, thus providing a cost advantage.

In accordance with a preferred embodiment of the present invention, theheating step preferably includes, after the piezoelectric thin film isdivided from the piezoelectric substrate by heating at a firsttemperature, an annealing step of annealing the piezoelectric thin filmat a second temperature greater than the first temperature.

The annealing of the piezoelectric thin film formed by dividing thepiezoelectric substrate at the portion having a peak concentration ofthe implanted element can remove ions remaining between crystal latticesand reduce a crystal lattice distortion caused by ion implantation, thusrecovering the crystallinity of the piezoelectric thin film. Thisrecovers the piezoelectricity of the piezoelectric thin film. Thus, apiezoelectric device having improved characteristics can bemanufactured.

In accordance with a preferred embodiment of the present invention, thesupporting substrate preferably includes a dielectric film disposed onthe piezoelectric thin film.

For example, in filters, a dielectric film may preferably be disposed ona piezoelectric thin film to control the propagation velocity of a waveor improve the reliability of the device. The material of such adielectric film must have a dielectric constant suitable for eachpurpose. However, under the constraint on thermal stress, the selectionof such a material has been significantly limited. In accordance withthis preferred embodiment, the supporting substrate including thedielectric film is formed on the piezoelectric thin film after theheating step. Thus, a material having any coefficient of linearexpansion can be used for the dielectric film without considering thethermal stress at the interface between the piezoelectric thin film andthe dielectric film.

In accordance with a preferred embodiment of the present invention, theelectrode forming step preferably includes forming an interdigitaltransducer (IDT) electrode as the functional electrode of thepiezoelectric device.

This enables the composite piezoelectric substrate to be used as asurface acoustic wave device or a boundary wave device, for example.

In accordance with a preferred embodiment of the present invention,preferably, the supporting substrate includes a membrane layer on thepiezoelectric thin film, the membrane layer including a sacrificiallayer region and a support layer region, and the method furtherincludes, after the supporting step, a step of removing the sacrificiallayer region to form a cavity.

In the piezoelectric thin film formed by dividing the piezoelectricsubstrate at the portion having a peak concentration of the implantedionized element, the collision energy of ions to be implanted and thelattice interval tends to increase with decreasing distance from theion-implanted surface. Thus, the piezoelectric thin film has a concavestress on the divided surface side and a convex stress on theion-implanted surface side. In conventional piezoelectric thin films,the ion-implanted surface of the piezoelectric thin film faces a cavityof a membrane structure, and the piezoelectric thin film tends to bulgeout on the cavity side, which causes sticking that tends to collapse thecavity. As described above, the front and back sides of thepiezoelectric thin film according to preferred embodiments are onopposite sides to the front and back sides of conventional piezoelectricthin films. The piezoelectric thin film according to preferredembodiments therefore tends to bulge out away from the cavity of themembrane structure, thus rarely causing sticking. Thus, a piezoelectricdevice having consistent characteristics can be manufactured.

In accordance with a preferred embodiment of the present invention, apiezoelectric device preferably includes a piezoelectric thin filmformed by dividing a piezoelectric substrate at a portion having a peakconcentration of an ionized element implanted in the piezoelectricsubstrate, a supporting substrate disposed on a side of the dividedsurface of the piezoelectric thin film, and a functional electrodedisposed on a side of the ion-implanted surface of the piezoelectricthin film.

In accordance with a preferred embodiment of the present invention, thefunctional electrode is preferably an interdigital transducer (IDT)electrode, for example.

In accordance with a preferred embodiment of the present invention, asupport layer region and a cavity are preferably disposed between thesupporting substrate and the piezoelectric thin film, the support layerregion enabling the piezoelectric thin film to be supported by thesupporting substrate, the cavity being formed by removal of asacrificial layer region.

In accordance with various preferred embodiments of the presentinvention, the temporary supporting substrate produces a negligiblethermal stress at the interface between the temporary supportingsubstrate and the piezoelectric substrate. Alternatively, the temporarysupporting substrate produces a thermal stress at the interface betweenthe temporary supporting substrate and the piezoelectric substrate lessthan the thermal stress at the interface between the supportingsubstrate and the piezoelectric substrate. Since the heating step isperformed while the temporary supporting substrate is disposed on a sideof the ion-implanted surface of the piezoelectric substrate, thepiezoelectric thin film includes fewer defects during heating thanconventional piezoelectric thin films. In particular, the compositepiezoelectric substrate can be manufactured with much fewer defectsusing a piezoelectric single crystal material. The supporting substrateis formed on the piezoelectric thin film after the heating step. Thus,material having any coefficient of linear expansion can be used for thesupporting substrate without considering the thermal stress at theinterface between the piezoelectric thin film and the supportingsubstrate in the heating step.

This enables various combinations of the material of the piezoelectricthin film and the material of the supporting substrate to be used. Inthe case of devices for filter applications, use of a supportingsubstrate made of a material having a much smaller coefficient of linearexpansion than a piezoelectric thin film improves thetemperature-frequency characteristics of a filter. Use of a materialhaving high thermal conductivity for the supporting substrate improvesthe heat radiation characteristics and the electric power resistance ofthe supporting substrate. Use of an inexpensive material for thesupporting substrate reduces the manufacturing costs of the device.Furthermore, use of a material having high processibility, such assilicon, for example, for a supporting substrate enables the supportingsubstrate to have a complicated structure. This permits a method forforming a piezoelectric thin film by dividing a piezoelectric substrateat a portion having a peak concentration of the implanted element to beapplied to various devices, such as micro-electro-mechanical systems(MEMS) and gyros, for example.

Piezoelectric degradation is less in the vicinity of the ion-implantedsurface of the piezoelectric thin film than in the vicinity of thedivided surface. Thus, a piezoelectric device having improvedcharacteristics can be manufactured by forming a functional electrode ona side of the ion-implanted surface.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method for manufacturing a piezoelectricdevice according to a first preferred embodiment of the presentinvention.

FIG. 2 is a schematic view of a process for manufacturing apiezoelectric device in accordance with the production flow illustratedin FIG. 1.

FIG. 3 is a schematic view of a process for manufacturing apiezoelectric device in accordance with the production flow illustratedin FIG. 1.

FIG. 4A is a schematic view of a surface acoustic wave devicemanufactured by a conventional method.

FIG. 4B is a schematic view of a surface acoustic wave device accordingto a preferred embodiment of the present invention.

FIG. 5 is a flow chart of a method for manufacturing a piezoelectricdevice according to a second preferred embodiment of the presentinvention.

FIG. 6 is a schematic view of a process for manufacturing apiezoelectric device in accordance with the production flow illustratedin FIG. 5.

FIG. 7 is a schematic view of a process for manufacturing apiezoelectric device in accordance with the production flow illustratedin FIG. 5.

FIG. 8A is a schematic view of a bulk wave device manufactured by aconventional method.

FIG. 8B is a schematic view of a bulk wave device according to apreferred embodiment of the present invention.

FIG. 9 is a flow chart of a method for manufacturing a piezoelectricdevice according to a third preferred embodiment of the presentinvention.

FIG. 10 is a schematic view of a process for manufacturing apiezoelectric device in accordance with the production flow illustratedin FIG. 9.

FIG. 11 is a schematic view of a process for manufacturing apiezoelectric device in accordance with the production flow illustratedin FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Preferred Embodiment

A method for manufacturing a surface acoustic wave device including acomposite piezoelectric substrate according to a first preferredembodiment of the present invention will be described below withreference to FIGS. 1 to 4B. FIG. 1 is a flow chart of a method formanufacturing a surface acoustic wave device according to the firstpreferred embodiment. FIGS. 2 and 3 are schematic cross-sectional viewsof manufacturing steps described in the flow chart in FIG. 1.

First, an ion implantation step is performed on a piezoelectricsubstrate 1 (FIGS. 1 and 2: S101). The piezoelectric substrate 1 ispreferably a single-crystal substrate made of a piezoelectric substance,for example. In the ion implantation step, ions are implanted into aflat main surface of the piezoelectric substrate 1 to form anion-implanted portion 2 in the main surface.

The piezoelectric substrate 1 is preferably a LiTaO₃ (LT) single-crystalsubstrate, for example. The piezoelectric substrate 1 can be divided bythe ion-implanted portion 2 by the application of heat as describedbelow. The piezoelectric substrate 1 may be made of any piezoelectricmaterial, such as LT, LiNbO₃ (LN), Li₂B₄O₇ (LBO), langasite(La₃Ga₅SiO₁₄), KNbO₃ (KN), or K₃Li₂Nb₅O₁₅ (KLN), for example.

In the first preferred embodiment, hydrogen ions are preferablyimplanted into the piezoelectric substrate 1 at an acceleration energyof approximately 150 KeV and a dose of approximately 1.0×10¹⁷ atom/cm²,for example. The hydrogen ions in the piezoelectric substrate 1 areprimarily distributed at a depth of approximately 1 μm, for example fromthe ion-implanted surface, forming the ion-implanted portion 2.Therefore, the ion-implanted portion 2 has the peak hydrogen ionconcentration. The type of ions to be implanted depends on the materialof the piezoelectric substrate 1 and may be helium ions or argon ions,for example. The ion implantation conditions depend on the material ofthe piezoelectric substrate 1 and the thickness of a piezoelectric thinfilm. For example, an acceleration energy of approximately 75 KeVresults in the ion-implanted portion 2 at a depth of approximately 0.5μm.

An etching layer forming step is then performed in which a layer to beetched 3 is formed on the ion-implanted surface of the piezoelectricsubstrate 1 (FIGS. 1 and 2: S102). The layer to be etched 3 maypreferably be made of a material that can be selectively etched in asubsequent temporary supporting substrate removing step while thepiezoelectric thin film and a supporting substrate are not etched. Thelayer to be etched 3 may preferably be made of an inorganic material,such as ZnO, SiO₂, or AlN, a metallic material, such as Cu, Al, or Ti,an organic material, such as polyimide, or a combination thereof, forexample. The layer to be etched may be omitted.

A temporary supporting step is then performed in which the layer to beetched 3 on the piezoelectric substrate 1 is bonded to a temporarysubstrate 4 (FIGS. 1 and 2: S103). The temporary substrate 4 and thelayer to be etched 3 define a temporary supporting substrate. Thematerial of the temporary supporting substrate produces a thermal stressat the interface between the temporary supporting substrate and thepiezoelectric substrate 1 that is less than the thermal stress at theinterface between a supporting substrate described below and thepiezoelectric substrate 1. Preferably, the material of the temporarysupporting substrate produces a negligible thermal stress.

In the first preferred embodiment, the temporary substrate 4 ispreferably an LT substrate, for example, which is the same as thepiezoelectric substrate 1, and the layer to be etched 3 is preferablycomposed of a Cu film and a SiO₂ film layered by sputtering, forexample. Thus, the temporary substrate 4 preferably has the same orsubstantially the same coefficient of linear expansion as thepiezoelectric substrate 1. This results in a negligible thermal stressat the interface between the piezoelectric thin film 11 and thetemporary supporting substrate, which is composed of the layer to beetched 3 and the temporary substrate 4. The layer to be etched 3 has acoefficient of linear expansion different from the coefficient of linearexpansion of the LT substrate. However, the formation of a film made ofa ductile material, such as a metallic material, for example, a Cu film,directly on the piezoelectric substrate 1 and a sufficient reduction inthe thickness of the layer to be etched 3 (approximately one-tenth orless of the thickness of the piezoelectric substrate or the temporarysubstrate) can reduce the thermal stress at the interface between thelayer to be etched 3 and the piezoelectric thin film 11.

A heating step is then performed in which the piezoelectric substrate 1,the layer to be etched 3, and the temporary substrate 4 are heated(FIGS. 1 and 2: S104). The heating enables the piezoelectric substrate 1to be divided by the ion-implanted portion 2 to form the piezoelectricthin film 11, thereby forming a composite piezoelectric substrateincluding the temporary substrate 4, the layer to be etched 3, and thepiezoelectric thin film 11. The composite piezoelectric substrate isthen annealed to recover its piezoelectricity at a temperature greaterthan the dividing temperature, for example, approximately 400° C., atwhich the piezoelectric substrate 1 has been divided to form thepiezoelectric thin film 11. For example, the annealing conditions maypreferably be approximately 500° C. for approximately three hours. Thus,the heating step including annealing can divide the piezoelectricsubstrate 1 by the ion-implanted portion 2 to form the piezoelectricthin film 11, remove hydrogen ions remaining between crystal lattices,reduce a crystal lattice distortion caused by ion implantation torecover the crystallinity of the piezoelectric thin film, and therebyrecover the piezoelectricity of the piezoelectric thin film.

A planarization and dielectric film forming step is performed in whichthe surface of the piezoelectric thin film 11 is planarized, and adielectric film 12 is formed on the piezoelectric thin film 11 (FIGS. 1and 2: S105). This step may be performed by any method at a temperaturesubstantially equal to or less than the annealing temperature,preferably substantially equal to or less than the dividing temperature.The dielectric film 12 is not essential and may be omitted.

The divided surface of the piezoelectric thin film 11 is preferablypolished, for example, by chemical-mechanical polishing (CMP) to asurface roughness Ra of approximately 1 nm or less. The dielectric film12 may preferably be made of silicon oxide, silicon nitride, siliconoxynitride, a metal oxide, a metal nitride, or diamond-like carbon, forexample. This enables the propagation velocity of a surface acousticwave in a surface acoustic wave device to be controlled in a manner thatdepends on the dielectric constant of the dielectric film 12, thuseasily achieving desired device characteristics. Preferably, thedielectric film 12 is made of a material having a large thermalconductivity and a small coefficient of linear expansion, as well as anappropriate dielectric constant. The dielectric film 12 may preferablyhave a multilayer structure, such as a two-layer structure including alayer having a small coefficient of linear expansion and a layer havinga large thermal conductivity. A large thermal conductivity improves theheat radiation characteristics and the electric power resistance of thesurface acoustic wave device. A small coefficient of linear expansionimproves the temperature-frequency characteristics of the surfaceacoustic wave device.

A supporting step is then performed in which the dielectric film 12 onthe piezoelectric thin film 11 is bonded to a base substrate 13 (FIGS. 1and 2: S106). This step may be performed by any method at a temperaturesubstantially equal to or less than the annealing temperature,preferably substantially equal to or less than the dividing temperature.

The base substrate 13 and the dielectric film 12 define a supportingsubstrate. Unlike the temporary supporting substrate, the supportingsubstrate may be made of a material having any coefficient of linearexpansion without considering the thermal stress at the interfacebetween the supporting substrate and the piezoelectric substrate(piezoelectric thin film) in the heating step. Thus, the dielectric film12 and the base substrate 13 may be made of a material having a muchsmaller coefficient of linear expansion than the piezoelectric thin film11. This significantly improves the temperature-frequencycharacteristics of the surface acoustic wave device. Use of aheat-conductive material for the dielectric film 12 and the basesubstrate 13 improves the heat radiation characteristics and theelectric power resistance of the surface acoustic wave device. Use of aninexpensive material for the dielectric film 12 and the base substrate13 and an inexpensive method for forming the dielectric film 12 and thebase substrate 13 reduces the manufacturing costs of the surfaceacoustic wave device.

A temporary supporting substrate removing step is then performed inwhich the temporary supporting substrate including the layer to beetched 3 and the temporary substrate 4 is removed (FIGS. 1 and 3: S107).This step may be performed by any method at a temperature substantiallyequal to or less than the annealing temperature, preferablysubstantially equal to or less than the dividing temperature.

In order to remove the temporary supporting substrate including thelayer to be etched 3 and the temporary substrate 4, the layer to beetched 3 is preferably wet-etched or dry-etched. In general, the layerto be etched 3 that is made of an inorganic material or a metallicmaterial is wet-etched, and the layer to be etched 3 that is made of anorganic material is dry-etched. Etching removes the layer to be etched 3and the temporary substrate 4 without unnecessary stress or impact onthe piezoelectric thin film 11, thereby reducing defects in thepiezoelectric thin film 11. The temporary substrate 4 separated from thelayer to be etched 3 is preferably reused to manufacture another surfaceacoustic wave device.

An electrode and protective film forming step is then performed in whichIDT electrodes 14, a lead wire 16, and a SiO₂ film defining an IDTelectrode protective film 15 are preferably formed on the piezoelectricthin film 11 (FIGS. 1 and 3: S108). The IDT electrodes 14 preferablyinclude two interdigitated electrodes, for example, and may preferablybe made of Al, W, Mo, Ta, Hf, Cu, Pt, Ti, and/or Au alone or incombination, for example. The IDT electrodes 14 may preferably be madeof an alloy thereof, for example. The lead wire 16 may preferably bemade of Al or Cu, for example. This step may be performed by any methodat a temperature substantially equal to or less than the annealingtemperature, preferably substantially equal to or less than the dividingtemperature.

An external terminal forming and dicing step is then performed in whichbumps 17 and solder balls 18 are formed, and a composite piezoelectricsubstrate including the piezoelectric thin film 11, the dielectric film12, and the base substrate 13 is divided into a plurality of surfaceacoustic wave devices 10 (FIGS. 1 and 3: S109). This step may beperformed by any method at a temperature substantially equal to or lessthan the annealing temperature, preferably substantially equal to orless than the dividing temperature.

Through these steps of manufacturing surface acoustic wave devices 10, acomposite piezoelectric substrate is formed. The composite piezoelectricsubstrate preferably includes the piezoelectric thin film 11 on thesupporting substrate including the dielectric film 12 and the basesubstrate 13. The piezoelectric thin film 11 is preferably formed by amethod for forming a piezoelectric thin film by dividing a piezoelectricsubstrate at an ion-implanted portion. In the surface acoustic wavedevice 10 manufactured using a temporary supporting substrate thatproduces a reduced thermal stress in the heating step, the piezoelectricthin film 11 has fewer defects resulting from thermal stress thanconventional piezoelectric thin films. Since the dielectric film 12 andthe base substrate 13 are formed on the piezoelectric thin film 11 afterthe heating step, the materials of the dielectric film 12 and the basesubstrate 13 can be determined without considering thermal stress in theheating step.

A single-crystal piezoelectric thin film prepared by dividing asingle-crystal substrate at an ion-implanted portion tends to be cleavedmore easily than piezoelectric thin films formed by deposition. Thus, ithas previously been essential to pay particular attention to stressresulting from a difference in the coefficient of linear expansion inthe heat treatment for recovering piezoelectricity. However, inaccordance with preferred embodiments of the present invention, asingle-crystal piezoelectric thin film can be easily manufactured whilereducing the stress resulting from a difference in the coefficient oflinear expansion.

The supporting substrate including the dielectric film and the basesubstrate 13 is preferably disposed on the divided surface of thepiezoelectric thin film 11 in the piezoelectric substrate 1. Thus, thedivided surface and ion-implanted surface of the piezoelectric thin film11 are on opposite sides to those manufactured by a conventional method.An example of the operational advantage of preferred embodiments of thepresent invention will be described below with reference to FIGS. 4A and4B based on the structure of the surface acoustic wave device 10 inwhich the divided surface and ion-implanted surface of the piezoelectricthin film 11 are on opposite sides to those manufactured by aconventional method.

FIG. 4A is a schematic view of a surface acoustic wave devicemanufactured by a conventional method. FIG. 4B is a schematic view of asurface acoustic wave device according to a preferred embodiment of thepresent invention. In both cases, ions are implanted through anion-implanted surface A into a piezoelectric substrate 1 in an ionimplantation step to form an ion-implanted portion 2. The piezoelectricsubstrate 1 is then divided at the ion-implanted portion 2 to form apiezoelectric thin film 11. In the conventional method, a supportingsubstrate is formed on the ion-implanted surface A, and a functionalelectrode of a device is formed on the divided surface B of thepiezoelectric thin film 11. In the first preferred embodiment of thepresent invention, a temporary supporting substrate is formed on theion-implanted surface A, a supporting substrate is formed on the dividedsurface B, and after the temporary supporting substrate is removed, afunctional electrode of a device is formed on the ion-implanted surfaceA.

The piezoelectric thin film 11 formed by a method for forming apiezoelectric thin film by dividing a piezoelectric substrate at anion-implanted portion includes some hydrogen ions even after thecrystallinity and the piezoelectricity of the piezoelectric thin filmhave been recovered in the heating step. The density of residualhydrogen ions is relatively high in the ion-implanted portion 2 in thepiezoelectric substrate 1, that is, in the vicinity of the dividedsurface B of the piezoelectric thin film 11, and relatively low in thevicinity of the ion-implanted surface A. Local piezoelectric degradationtends to decrease with decreasing density of residual hydrogen ions.Thus, piezoelectric degradation is relatively large in the vicinity ofthe divided surface B and relatively small in the vicinity of theion-implanted surface A. A surface acoustic wave device according to thepresent preferred embodiment of the present invention preferablyincludes a functional electrode on the ion-implanted surface A whichsuffers less piezoelectric degradation. Thus, the surface acoustic wavedevice according to the present preferred embodiment has bettercharacteristics than a surface acoustic wave device manufactured by aconventional method.

Although a method for manufacturing a surface acoustic wave device hasbeen described in the first preferred embodiment, an elastic boundarywave device can be manufactured by the same process.

Second Preferred Embodiment

A method for manufacturing a bulk wave device including a compositepiezoelectric substrate according to a second preferred embodiment ofthe present invention will be described below with reference to FIGS. 5to 8B. FIG. 5 is a flow chart of a method for manufacturing a bulk wavedevice according to the second preferred embodiment. FIGS. 6 and 7 areschematic cross-sectional views of manufacturing steps described in theflow chart in FIG. 5.

In the same or substantially the same manner as in the first preferredembodiment, an ion implantation step is performed first (FIG. 5: S201).An etching layer forming step is then performed (FIG. 5: S202). Atemporary supporting step is then performed (FIG. 5: S203). A heatingstep is then performed (FIGS. 5 and 6: S204). A temporary supportingsubstrate including a layer to be etched 23 and a temporary substrate 24is formed on the ion-implanted surface of a piezoelectric thin film 31preferably having a thickness of approximately 1 μm, for example. As inthe first preferred embodiment, the layer to be etched may be omitted.

The divided surface of the piezoelectric thin film 31 is planarized, andlower electrodes 34, e.g., a lower drive electrode and a lower leadwire, arranged to drive a bulk wave device are formed (FIGS. 5 and 6:S205). This step may be performed by any method at a temperaturesubstantially equal to or less than the annealing temperature,preferably substantially equal to or less than the dividing temperature.

The material of the lower drive electrode depends on the requiredphysical properties of the bulk wave device and may preferably be ahigh-melting-point metallic material, such as W, Mo, Ta, or Hf, alow-resistance metal, such as Cu or Al, a metallic material resistant tothermal diffusion, such as Pt, or a metallic material having goodadhesion to a piezoelectric material, such as Ti or Ni, for example. Thelower drive electrode may preferably be a multilayer film including atleast one of these metallic materials. The electrode may be formed byany method depending on the type of the electrode material and thedesired physical properties, for example, electron-beam (EB) vapordeposition, sputtering, or chemical vapor deposition (CVD). The materialof the lower lead wire may preferably be Cu or Al, for example.

A sacrificial layer region 32A having a membrane structure is thenformed (FIGS. 5 and 6: S206). This step may be performed by any methodat a temperature substantially equal to or less than the annealingtemperature, preferably substantially equal to or less than the dividingtemperature.

A support layer region 32B having a membrane structure is then formed(FIGS. 5 and 6: S207). This step may be performed by any method at atemperature substantially equal to or less than the annealingtemperature, preferably substantially equal to or less than the dividingtemperature. The surface of the support layer region 32B is preferablypolished to planarize the surface.

The sacrificial layer region 32A and the support layer region 32B eachpreferably having a membrane structure define a membrane layer. Thematerial of the sacrificial layer region 32A is preferably chosen suchthat an etching gas or an etching liquid in the subsequent removal ofthe sacrificial layer can etch the sacrificial layer region 32A butcannot etch the lower electrodes 34. The material of the sacrificiallayer may preferably be a metal, such as Ni, Cu, or Al, an insulatingmaterial, such as SiO₂, ZnO, or phosphosilicate glass (PSG), or anorganic material, for example.

The membrane layer including the sacrificial layer region 32A and thesupport layer region 32B is bonded to a base substrate 33 (FIGS. 5 and6: S208). This step may be performed by any method at a temperaturesubstantially equal to or less than the annealing temperature,preferably substantially equal to or less than the dividing temperature.

The base substrate 33 and the membrane layer define a supportingsubstrate. The supporting substrate including the base substrate 33, thesacrificial layer region 32A, and the support layer region 32B may bemade of a material having any coefficient of linear expansion withoutconsidering the thermal stress at the interface between the supportingsubstrate and the piezoelectric thin film 31.

The layer to be etched 23 is then etched to remove the temporarysupporting substrate including the layer to be etched 23 and thetemporary substrate 24 (FIGS. 5 and 7: S209). This step may be performedby any method at a temperature substantially equal to or less than theannealing temperature, preferably substantially equal to or less thanthe dividing temperature.

Upper electrodes 35, including an upper drive electrode and an upperlead wire, to drive the bulk wave device are preferably formed on theion-implanted surface of the piezoelectric thin film 31 (FIGS. 5 and 7:S210). This step may be performed by any method at a temperaturesubstantially equal to or less than the annealing temperature,preferably substantially equal to or less than the dividing temperature.A plurality of windows 36 are preferably bored in the piezoelectric thinfilm 31 to provide an etchant inlet for the removal of the sacrificiallayer and a lower electrode wire outlet. A via electrode and a padelectrode are preferably formed in the lower electrode wire outlet. Thewindows are preferably formed by patterning of a resist byphotolithography and dry etching using the resist as a mask. In the samemanner as in the lower electrode material, the material of the upperelectrodes 35 depends on the required physical properties of the bulkwave device.

An etchant is introduced into the sacrificial layer region 32A throughthe windows 36. The material of the sacrificial layer region 32A ispreferably removed to form a cavity 38 of a membrane structure (FIGS. 5and 7: S211). Wet etching or dry etching is selected based on thematerial of the sacrificial layer region 32A. Wet etching liquid andetching gas should not affect the piezoelectric thin film 31 and thelower and upper electrodes 34 and 35.

External terminals 39 each including a bump 17 and a solder ball 39 arethen formed. A composite piezoelectric substrate including thepiezoelectric thin film 31, the support layer region 32B, and the basesubstrate 33 is divided into a plurality of bulk wave devices 30 (FIGS.5 and 7: S212).

Through these steps of manufacturing bulk wave devices 30, a compositepiezoelectric substrate is formed. The composite piezoelectric substrateincludes the piezoelectric thin film 31 on the supporting substrateincluding the support layer region 32B and the base substrate 33. Thepiezoelectric thin film 31 is preferably formed by a method for forminga piezoelectric thin film by dividing a piezoelectric substrate at anion-implanted portion. In the bulk wave device 30 manufactured using atemporary supporting substrate that can produce a reduced thermal stressin the heating step, the piezoelectric thin film 31 has fewer defectsresulting from thermal stress than conventional piezoelectric thinfilms. Since the sacrificial layer region 32A, the support layer region32B, and the base substrate 33 are formed on the piezoelectric thin film31 after the heating step, the materials of the sacrificial layer region32A, the support layer region 32B, and the base substrate 33 can bedetermined without considering thermal stress in the heating step.

The supporting substrate including the support layer region 32B and thebase substrate 33 is disposed on the divided surface of thepiezoelectric thin film 31. Thus, the divided surface and ion-implantedsurface of the piezoelectric thin film 31 are on opposite sides to thosemanufactured by a conventional method. An example of the operationaladvantage of a preferred embodiment of the present invention will bedescribed below with reference to FIGS. 8A and 8B based on the structureof the bulk wave device 30 in which the divided surface andion-implanted surface of the piezoelectric thin film 31 are on oppositesides to those manufactured by a conventional method.

FIG. 8A is a schematic view of a bulk wave device manufactured by aconventional method. FIG. 8B is a schematic view of a bulk wave deviceaccording to a preferred embodiment of the present invention. In bothcases, ions are implanted through an ion-implanted surface A into apiezoelectric substrate 21 in an ion implantation step to form anion-implanted portion 22. The piezoelectric substrate 21 is then dividedat the ion-implanted portion 22 to form a piezoelectric thin film 31. Inaccordance with a conventional method, a membrane layer is formed on theion-implanted surface A to form a cavity 38 of a membrane structure. Inaccordance with the present preferred embodiment of the presentinvention, after a temporary supporting substrate is formed on theion-implanted surface A, a membrane layer is formed on the dividedsurface B to form a cavity 38 of a membrane structure.

In the piezoelectric thin film 31 formed by a method for forming apiezoelectric thin film by dividing a piezoelectric substrate at anion-implanted portion, the collision energy of hydrogen ions and thelattice interval increase with decreasing distance from theion-implanted surface A. Thus, the piezoelectric thin film has a concavestress on the divided surface side and a convex stress on theion-implanted surface side. Therefore, in the bulk wave device accordingto the present preferred embodiment of the present invention, theion-implanted surface bulges out away from the cavity of the membranestructure, thus causing sticking which collapses the cavity much lessfrequently than a bulk wave device manufactured by a conventionalmethod. Thus, the bulk wave device according to the present preferredembodiment has a highly reliable structure.

The manufacturing method according to the second preferred embodiment isapplicable to other devices having a membrane structure, such as filmbulk acoustic resonator (FBAR) devices, plate wave devices, and Lambwave devices, for example.

Third Preferred Embodiment

A method for manufacturing a gyro device including a compositepiezoelectric substrate according to a third preferred embodiment of thepresent invention will be described below with reference to FIGS. 9 to11. A gyro device vibrates a silicon diaphragm with a piezoelectricvibrator, and detects the Coriolis force acting on the silicon diaphragmduring rotation with the piezoelectric vibrator. A method formanufacturing a tuning-fork gyro device will be described below. Thetuning-fork gyro device includes a piezoelectric vibrator and a silicondiaphragm. The piezoelectric vibrator includes a piezoelectric thin filmand a drive electrode. The piezoelectric vibrator is disposed on thesilicon diaphragm.

FIG. 9 is a flow chart of a portion of a method for manufacturing a gyrodevice according to the third preferred embodiment. FIGS. 10 and 11 areschematic cross-sectional views of manufacturing steps described in theflow chart in FIG. 9.

In substantially the same manner as in the first and second preferredembodiments, an ion implantation step is performed first (FIG. 9: S301).An etching layer forming step is then performed (FIG. 9: S302). Atemporary supporting step is then performed (FIG. 9: S303). A heatingstep is then performed (FIGS. 9 and 10: S304). A temporary supportingsubstrate including a layer to be etched 43 and a temporary substrate 44is formed on the ion-implanted surface of a piezoelectric thin filmpreferably having a thickness of approximately 1 μm, for example. Thelayer to be etched may be omitted.

The divided surface of the piezoelectric thin film 51 is preferablyplanarized, a lower electrode 54 to drive vibrators 50A described beloware formed, and an insulating layer 56 is formed on the lower electrode54 (FIGS. 9 and 10: S305). The insulating layer 56 defines a bondinglayer between the vibrators 50A and a silicon diaphragm 50B. This stepmay be performed by any method at a temperature substantially equal toor less than the annealing temperature, preferably substantially equalto or less than the dividing temperature.

The material of the lower electrode 54 is selected based on on therequired physical properties of the gyro device and may preferably be ahigh-melting-point metallic material, such as W, Mo, Ta, or Hf, alow-resistance metal, such as Cu or Al, a metallic material resistant tothermal diffusion, such as Pt, or a metallic material having goodadhesion to a piezoelectric material, such as Ti or Ni, for example. Thelower electrode 54 may preferably be a multilayer film including atleast one of these metallic materials, for example. The insulating layer56 may preferably be formed of silicon oxide or silicon nitride, forexample.

The insulating layer 56 is then bonded to the silicon diaphragm 50B(FIGS. 9 and 10: S306). This step may be performed by any method at atemperature substantially equal to or less than the annealingtemperature, preferably substantially equal to or less than the dividingtemperature.

The silicon diaphragm 50B, the lower electrode 54, and the insulatinglayer 56 define a supporting substrate. The supporting substrate (thelower electrode 54, the insulating layer 56, and the silicon diaphragm50B) may be made of a material having any coefficient of linearexpansion without considering the thermal stress at the interfacebetween the supporting substrate and the piezoelectric thin film 51.

The layer to be etched 43 is then etched to remove the temporarysupporting substrate including the layer to be etched 43 and thetemporary substrate 44 (FIGS. 9 and 10: S307). This step may beperformed by any method at a temperature substantially equal to or lessthan the annealing temperature, preferably substantially equal to orless than the dividing temperature.

An upper electrode 55 to drive the gyro device is then formed on theion-implanted surface of the piezoelectric thin film 51. The upperelectrode 55, the piezoelectric thin film 51, and the lower electrode 54are patterned in a predetermined shape to form the vibrators 50A (FIGS.9 and 10: S308). This step may be performed by any method at atemperature substantially equal to or less than the annealingtemperature, preferably substantially equal to or less than the dividingtemperature. In substantially the same manner as in the lower electrodematerial, the material of the upper electrode 55 is selected based onthe required physical properties of the gyro device.

The silicon diaphragm 50B is then bonded to a support substrate 57 viaan adhesive (FIGS. 9 and 11: S309). This step may be performed by anymethod at a temperature substantially equal to or less than theannealing temperature, preferably substantially equal to or less thanthe dividing temperature.

The silicon diaphragm 50B is preferably dry-etched in a desired shape(tuning fork) with a fluorine-based gas, for example (FIGS. 9 and 11:S310). This step may be performed by any method at a temperaturesubstantially equal to or less than the annealing temperature,preferably substantially equal to or less than the dividing temperature.

The support substrate 57 is then removed to form gyro devices 50 eachincluding the vibrator 50A and the silicon diaphragm 50B (FIGS. 9 and11: S310).

Through these steps of manufacturing the gyro devices 50, a compositepiezoelectric substrate is formed. The composite piezoelectric substrateincludes the vibrator 50A on the silicon diaphragm 50B. The vibrator 50Aincludes the piezoelectric thin film 51 formed by a method for forming apiezoelectric thin film by dividing a piezoelectric substrate utilizingan ion-implanted portion. In the gyro device 50 manufactured using atemporary supporting substrate that can produce a reduced thermal stressin the heating step, the piezoelectric thin film 51 has fewer defectsresulting from thermal stress than conventional piezoelectric thinfilms. Since the silicon diaphragm 50B is formed on the vibrator 50Aafter the heating step, the material of the silicon diaphragm 50B can bedetermined without considering thermal stress in the heating step.

In substantially the same manner as this manufacturing method, variousMEMS devices, such as radio frequency (RF) switch devices, for example,can be manufactured. MEMS devices often include a silicon substrate as adevice supporting material in view of processibility. Since siliconsubstrates generally have a smaller coefficient of linear expansion thanpiezoelectric substrates (piezoelectric thin films), it is difficult toheat a silicon substrate to an annealing temperature by a conventionalmethod. In accordance with preferred embodiments of the presentinvention, MEMS devices including a silicon substrate as a supportingsubstrate can be manufactured without any problems.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A method for manufacturing a compositepiezoelectric substrate that includes a piezoelectric thin filmsupported by a supporting substrate, the method comprising: an ionimplantation step of implanting an ionized element into a surface of apiezoelectric substrate to form a portion of the piezoelectric substratehaving a peak concentration of the ionized element; a temporarysupporting step of forming a temporary supporting substrate on a side ofthe ion-implanted surface of the piezoelectric substrate, the temporarysupporting substrate being made of the same material as thepiezoelectric substrate or producing a thermal stress at an interfacebetween the temporary supporting substrate and the piezoelectricsubstrate that is less than a thermal stress at an interface between thesupporting substrate and the piezoelectric thin film; a heating step ofheating the piezoelectric substrate to divide the piezoelectric thinfilm from the piezoelectric substrate at the portion of thepiezoelectric substrate having the peak concentration of the ionizedelement, wherein the side of the ion-implanted surface of thepiezoelectric substrate is the same side of the ion-implanted surface ofthe piezoelectric thin film, and the piezoelectric thin film includesthe side of the ion-implanted surface of the piezoelectric thin film anda side opposite to the side of the ion-implanted surface of thepiezoelectric thin film; and a supporting step of forming the supportingsubstrate on the side opposite to the side of the ion-implanted surfaceof the piezoelectric thin film.
 2. The method for manufacturing acomposite piezoelectric substrate according to claim 1, wherein thesupporting step of forming the supporting substrate on the piezoelectricthin film further comprises, after the supporting step, a temporarysupporting substrate removing step of removing the temporary supportingsubstrate disposed on the side of the ion-implanted surface of thepiezoelectric thin film and an electrode forming step of forming afunctional electrode of a piezoelectric device on the side of theion-implanted surface.
 3. The method for manufacturing a compositepiezoelectric substrate according to claim 2, wherein the electrodeforming step includes forming an interdigital transducer electrode asthe functional electrode of the piezoelectric device.
 4. The method formanufacturing a composite piezoelectric substrate according to claim 1,wherein the temporary supporting substrate includes a layer to beetched, and the temporary supporting substrate removing step includesremoving the temporary supporting substrate by etching the layer to beetched.
 5. The method for manufacturing a composite piezoelectricsubstrate according to claim 1, wherein the heating step includes, afterthe piezoelectric thin film is divided from the piezoelectric substrateby heating at a first temperature, an annealing step of annealing thepiezoelectric thin film at a second temperature greater than the firsttemperature.
 6. The method for manufacturing a composite piezoelectricsubstrate according to claim 1, wherein the supporting substrateincludes a dielectric film disposed on the piezoelectric thin film. 7.The method for manufacturing a composite piezoelectric substrateaccording to claim 1, wherein the supporting substrate includes amembrane layer on the piezoelectric thin film, the membrane layerincluding a sacrificial layer region and a support layer region, andfurther including the supporting substrate, and the method furtherincludes, after the supporting step, a step of removing the sacrificiallayer region to form a cavity.